1. Field of the Invention
The present invention relates to surgical apparatus and more particularly to a hand held electrical stimulus probe for use as an intraoperative aid in defining the course of neural structures or in nerve integrity monitoring. The invention is particularly applicable for use in monitoring facial electromyogram (EMG) activity during surgeries in which a facial motor nerve is at risk due to unintentional manipulation and will be described with reference thereto, although it will be appreciated that the invention has broader applications and can be used in other neural monitoring procedures.
2. Discussion of the Prior Art
Despite advancements in diagnosis, microsurgical techniques, and neurotological techniques enabling more positive anatomical identification of facial nerves, loss of facial nerve function following head and neck surgery such as acoustic neuroma resection is a significant risk. Nerves are very delicate and even the best and most experienced surgeons, using the most sophisticated equipment known, encounter a considerable hazard that a nerve will be bruised, stretched or even severed during an operation.
Studies have shown that preservation of the facial nerve during acoustic neuroma resection may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Very broadly stated, the locating procedure, also known as nerve integrity monitoring, involves inserting sensing or recording electrodes directly within cranial muscles controlled by the nerve of interest. An electrical stimulation probe is then applied near the area where the subject nerve is believed to be located. If the stimulation probe contacts or is reasonably near the nerve, the stimulation signal applied to the nerve is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle; the impulse is transferred to the recording electrodes, thereby providing an indication to the surgeon as to the location of the nerve.
While intraoperative electrical stimulation has been of benefit in localization and preservation of facial nerves during various surgical procedures, the accuracy and reliability of the indication of stimulation depends upon eliminating sources of false indications of stimulation. A major source of false indications of stimulation is the shunting of the electrical stimulus current away from the intended area and through the body fluids. During acoustic neuroma surgery the surgical area is invariably bathed in cerebral spinal fluid (CSF), a clear, colorless body fluid containing electrolytes and capable of conducting electrical current. The earliest stimulus probes were crude segments of uninsulated wire or tapered metal rods touched to the area to be stimulated, often allowing electrical contact with CSF electrolyte fluid, whereupon the electrical stimulus current was allowed to spread along the shunt or parallel paths through the body.
Spreading of the stimulus current reduces the desired electrical current flowing through the nerve tissue intended for stimulation, which may result in false negative indications of stimulation and thus adversely effect the accuracy of the procedure. In the past, others have attempted to compensate for the problem of current stimulus spread by simply increasing the intensity level of the electrical stimulus, whereby the neural response to stimulation occurs despite the current shunted through undesired paths. Increased stimulus current levels increase the possibility of tissue damage, however. In addition, the increased stimulus current may also spread through undesired paths in inactive tissue, reaching the active nerve tissue at a level sufficient to produce a false positive response or indication of stimulation, thus affecting the accuracy of the procedure, as above.
One of the inventors of the present invention addressed current shunting problems in the Electrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (to Richard L. Prass), the entire disclosure of which is incorporated herein by reference. The probe of U.S. Pat. No. 4,892,105 has become known as the Prass Flush-Tip Monopolar Probe and is insulated up to the distal tip to minimize current shunting through undesired paths. The Prass Flush-Tip Monopolar Probe is difficult to use when it is desired to provide a bipolar stimulus, however. Bipolar stimulus is employed whenever it is desired to provide a current path from anode to cathode through desired nerve tissue and at controlled depth into the nerve tissue. In order to provide bipolar stimulus with the Prass Flush-Tip Monopolar Probe, an anode probe and a cathode probe must be placed on or near the nerve and held in place during stimulation; repeatable and consistent placement of the individual monopolar cathode and anode probes must be maintained in order to avoid changes in the detection of the stimulus current, possibly leading to a false response or indication of stimulation, thus affecting the accuracy of the procedure, as above.
Monopolar probes and Bipolar probes (having integral cathode and anode tips) are well suited to specific uses. For most applications of nerve integrity monitoring equipment, the flush tip monopolar probe (having only a cathode) is selected for initial stimulation of motor nerves. In operation, the current spreads out in all directions from the stimulating cathode contact and returns via an anode contact, usually a needle in the patient's shoulder. Current spread increases with increasing current levels and is likely to cause stimulation of any nearby nerve tissue even when the probe cathode contact is not actually touching the nerve or making particular good connection to the nerve. Greater specificity or spatial selectivity can be obtained by operating the probe at reduced levels of stimulus current. At small levels of stimulus current, the nerve is stimulated only when the probe is in direct contact with the neural structure. Accordingly, a balance must be struck since, as stimulation current is decreased, specificity is improved but it is more likely that insufficient current will be provided to stimulate the nerve. At moderate or high levels of monopolar current stimulation, current may be conducted through non-neural tissue at levels adequate to stimulate adjacent neural tissue, causing a false positive response. Moreover, stimulation current may travel through inactive (non-neural) tissue and stimulate motor nerve tissue or adjacent neural structures which may respond simultaneously with the desired neural structure, an undesirable result when seeking to identify a specific motor nerve. Monopolar stimulation at moderate to high current levels is therefore most useful when mapping the course of a selected motor nerve structure but is not well suited when seeking to stimulate a single selected motor nerve in an area of the body having many closely spaced nerve structures, in which case bipolar stimulation is more likely to be effective.
A bipolar stimulating probe offers increased specificity for differentiating adjacent neural structures at moderate to high stimulation current levels. The most important difference between the bipolar stimulation and monopolar stimulation is that current flows directly between cathode and anode tips mounted on the distal end of the bipolar probe instead of going from the monopolar probe cathode to a distant return anode while spreading in all directions from the probe tip. The bipolar probe design permits current flow only from the distal cathode tip to the distal anode tip and therefore primarily stimulates those neural structures between the cathode tip and anode tip. Accordingly, monopolar excitation is preferred when mapping or locating the trajectory of the motor nerves. Once a motor nerve is located, bipolar excitation is preferred for use in differentiating among adjacent nerves.
Others have developed bipolar probes providing an exposed anode conductive tip and an exposed cathode conductive tip, however, the existing bipolar stimulus probes have not proven entirely satisfactory.
Most bipolar probe tips are relatively large to accommodate both a cathode and anode electrode while allowing sufficient inter-electrode distance to ensure adequate penetration of stimulus current into the tissue; there is also a problem of handedness, meaning that a probe may be well suited for use by a left handed or right handed surgeon or for left or right sided surgery, but not both.
Since the insulated probe tips have a specific planar orientation, it may be difficult to accurately place both cathode and anode probe tips flush on the nerve for precisely targeted stimulation. Often, it is desired to malleably flex or plastically deform the probe tips into a more convenient orientation for a given tissue topography, and the bipolar probes of the prior art fail to maintain the preselected anode tip to cathode tip spacing after deformation, thus leading to loss of calibration in the stimulus current, especially in areas within the body having uneven topography.
Additionally, when performing surgery under a microscope it is difficult for the surgeon to determine which tip is the anode and which is the cathode, and so the surgeon may or may not know the direction of current flow during stimulation. The bipolar probes of the prior art also exhibit relatively low efficiency in stimulation of exposed nerve tissue, as compared to monopolar probes.
There is a need, therefore, for an improved method and apparatus for providing bipolar stimulation and/or sensing or recording of electrical activity in the nerve tissue.